A variety of electron microscopes and associated surface analyzers have evolved in recent years. A popular type is a scanning electron microscope in which a focused electron beam is scanned over a sample surface where secondary electrons are emitted and detected in correlation with scanning position. The secondary electrons are processed electronically to provide a picture of topographical features. Associated mapping of chemical constituents in the surface is achieved with characteristic x-rays produced by the electron beam.
Another method of measuring for constituents near the surface of a sample is electron spectroscopy for chemical analysis (ESCA) which involves irradiating a sample surface with x-rays and detecting the characteristic photoelectrons emitted. The photoelectrons are filtered by electrostatic or magnetic means to pass through electrons of a specified energy. The intensity of the filtered beam reflects the concentration of a given chemical constituent of the sample surface.
U.S. Pat. No. 3,617,741 (Siegbahn et al.) for example, teaches the use of a hemispherical electrostatic analyzer (SCA) for selectively filtering electron energy for ESCA. An outer hemisphere is maintained at a negative potential with respect to an inner concentric hemisphere so as to cause electrons entering the space between the hemispheres to follow curved trajectories according to electron energy. The 180.degree. (.pi. radians) trajectory defined by the hemispheres is especially desirable because the electrons exit the hemispheres in an image plane that correlates with the inlet image, providing for optimum energy resolution. The patent also discloses an input lens system for modifying the energy of the electrons entering the SCA.
Hemispherical analyzers are used similarly for analysis and spectroscopy with secondary Auger electrons generated at the sample surface by the focused primary electron beam. Auger microprobes are suitable for detecting elements with low atomic numbers and have sensitivity to a few atomic layers. Surface mapping of elements is accomplished by scanning with the primary electron beam.
Another electron optical system useful for filtering and spectroscopy utilizes a cylindrical mirror analyzer such as described in U.S. Pat. No. 4,048,498 (Gerlach et al.). In such an arrangement, concentric cylinders, with the outer being charged negatively with respect to the inner, refract diverging electron beams back to the axis of the cylinders and filter in a manner similar to the hemispherical analyzer. However, the cylindrical filter does not provide a very narrow band of energies, i.e. energy resolution.
A problem with the aforementioned hemispherical type of analyzer is that solid collection angle efficiency is relatively low and, also, the hemispherical analyzer is not efficiently used. In particular, charged particles traverse the spherical analyzer only in a small region, proximate a single plane intersecting the spherical center. An effort to expand the input solid angle of a spherical analyzer is described in "The Spherical Condenser as a High Transmission Particle Spectrometer" by R. H. Ritchie, J. S. Cheka and R. D. Birkhoff, Nuclear Instruments and Methods, Vol. 6, pages 157-163 (1960). A source of charged particles is placed on the inner sphere and charged particles follow trajectories in all directions through the volume between spheres. The particles exit in a conically converging pattern for detection. This system does not allow for any preliminary optics or filtering of the charged particles prior to energy analysis.
Efficient use of input solid angle is also described in "IEE--A New Type of X-ray Photoelectron Spectrometer" by N. H. Weichert and J. C. Helmer, Varian Associates, Palo Alto, Calif. Two concentric spherical electrodes in figure of rotation are described, the spheres being sectioned to receive particles from a sample on the axis of rotation. The particles pass through the analyzer and focus back to the axis where they are detected. This system is more versatile than that described by Ritchie et al.; however, the arrangement does not allow for the advantages of a 180.degree. path in the spherical analyzer. Such a 180.degree. path allows for electrons to originate a large distance off axis, thereby giving large luminosity (input area times solid angle) which is especially important for ESCA.
A similar device is described in "Novel Charged Particle Analyzer for Momentum Determination in the Multi-Channeling Mode" by H. A. Engelhardt, W. Back and D. Menzel, Review of Scientific Instruments. Vol. 52, pages 835-839 (1981). Trajectory angle is increased by bringing particles back to the detector at the axis perpendicularly. In this device, a truncated conical lens coaxial with the analyzer is utilized for retarding and focusing electrons into the analyzer from a sample surface at the axis.
In view of the foregoing, a primary objective of the present invention is to provide an energy analyzing system for charged particles with improved collection efficiency and energy resolution.
Another object is to provide a novel spherical capacitor energy analyzer for charged particles.
A further object is to provide a novel energy analyzer with both high luminosity and high input solid angle that is particularly useful for x-ray photoelectron chemical analysis of large or small surface areas.
Yet another object is to provide a novel energy analyzer with high input solid angle, that is particularly useful for Auger electrons.